1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording and reproducing information as hologram in and from an optical information recording medium in which the information is recorded as hologram, a diffraction-grating fabricating apparatus, the optical information recording medium, and a positioning control method.
2. Description of the Related Art
Optical information recording media include a compact disk (CD), a digital versatile disk (DVD), and a high-definition digital versatile disc (HD DVD). The optical information recording medium responds to an increase in recording density so far mainly by making a wavelength of a laser beam shorter and by increasing the number of apertures (NA) of an objective lens. However, both the methods seem to be approaching the limit by some technical reasons, and it is therefore required to increase the recording density by other means and systems.
Recently, among various approaches, a volume-recording type high-density optical recording using holography (hereinafter, “holographic memory”) and a recording/reproducing device of the holographic memory (hereinafter, “holographic-memory recording/reproducing device”) are being developed for practical use. A recording system of the holographic memory is implemented by irradiating an information beam and a reference beam to one location in a recording medium and by recording light interference fringes formed by the information beam and the reference beam upon the irradiation, in the recording medium. More specifically, the information beam carries information by spatially modulating a laser beam by a space modulator such as a liquid crystal element and a digital micromirror device, and the reference beam has the same wavelength as that of the information beam and is generated usually from the same light source as the information beam.
By irradiating only the reference beam to the holographic memory for reproduction, the recorded information beam is reproduced to obtain the information which has been modulated upon the recording. The DVD or the like is based on a so-called surface recording system of recording a recording mark on a recording surface, while a holographic optical disc is based on a volume recording system capable of recording information in a thickness direction of an information recording layer. Therefore, the holographic optical disc is expected to have a high recording density as compared with that of the DVD or the like.
In the case of the DVD or the like, the recording mark generally indicates bit data for on/off, while in the case of the holographic memory, the information beam is collectively modulated by a comparatively large amount of information and recorded as interference fringes. A set of information is a pattern of the information beam stored in the recording medium, and is a minimum unit of a two-dimensional bar code formed by black and white dots for recording and reproduction. The set of information is called page data.
There is a multiple recording system as one of the methods of increasing the recording density of the holographic memory. The multiple recording system is a system of recording a plurality of page data in one location of the holographic memory. The recording is represented by angular multiplexing recording such that an angle of irradiating a laser beam is shifted, and by shift multiplexing recording such that a position irradiated with a laser beam is slightly shifted.
In an angular multiplexing recording system and a shift multiplexing recording system, multiple recording is implemented by changing a relative position and a relative angle between the laser beam and the recording medium for the holographic memory (hereinafter, “holographic-memory recording medium”). Particularly, the angular multiplexing recording system is a completely different system from that of the conventional CD, DVD, and the like. If any one of the conventional ones is combined with the shift multiplexing recording system, typical two types of methods are considered. One of the methods is a technology (for example, U.S. Pat. No. 5,483,365) of performing multiple recording not by using a laser beam but by rotating a medium, and the other one is a technology (for example, U.S. Patent Application No. 2004/0179251) of performing multiple recording by rotating a laser beam around a medium.
The conventional technology disclosed in U.S. Pat. No. 5,483,365 has characteristics such that there is no need to provide a movable unit to perform multiple recording in an optical system such as a lens, and thus the configuration of a device can be made simple. However, this technology is difficult to be applied to a rotating disc such as CD and DVD. Conversely, the conventional technology disclosed in U.S. Patent Application No. 2004/0179251 has characteristics such that the configuration of a device becomes too large although this technology is easy to be applied to the rotating disc such as CD and DVD.
In both the conventional technologies, the multiple recording is an essential technology for the holographic memory, and thus it is necessary to establish a technology for detecting servo information indicating the relative position and the relative angle between the laser beam and the holographic-memory recording medium. This technology is an important task for practical use, and some technologies have already been developed.
As disclosed in, for example, JP-A 2003-178484 (KOKAI), JP-A 2003-228849 (KOKAI), JP-A 2004-265472 (KOKAI), and JP-A 2004-326897 (KOKAI), there is known a technology for providing a servo pit for controlling a position in a holographic-memory recording medium, and for detecting the position using a light reflected from the servo pit.
As disclosed in JP-A 2006-171589 (KOKAI) and JP-A 2006-171593 (KOKAI), there is known a method of providing a diaphragm between a holographic-memory recording medium and an imaging device provided downstream thereof, detecting light cut by a pinhole to detect and adjust a displacement of a medium upon reproduction of information.
However, in the conventional technologies disclosed in JP-A 2003-178484 (KOKAI), JP-A 2003-228849 (KOKAI), JP-A 2004-265472 (KOKAI), JP-A 2004-326897 (KOKAI), JP-A 2006-171589 (KOKAI), and JP-A 2006-171593 (KOKAI), some of the inventions are difficult to be applied to a case where the invention is based on a reflective type holographic-memory recording medium or to a case where an incident angle of a laser beam on the holographic-memory recording medium is large. Further, in these conventional technologies, there are problems in which the holographic-memory recording medium requires a filter layer to separate a servo laser beam, detection is limited to a case where information is reproduced, and it is difficult to detect a displacement between an information beam and a reference beam.